How to Calculate Capacity or Size Storage
1. Choose a workflow
Choose existing bunker capacity or herd-based bunker sizing.
2. Choose forage and units
Select a forage preset and units, then replace preset dry matter and density with measured values when available.
3. Enter dimensions or demand
For capacity, select the bunker shape and enter internal dimensions. For sizing, enter direct daily silage demand or use the advanced ration method.
4. Review the result
Calculate and review volume, tonnage range, dry matter, usable inventory, feeding days, face advance, dimensions, and packing guidance.
Formulas and Assumptions
Rectangular volume = floor width × packed depth × usable length
Tapered volume = (floor width + top width) ÷ 2 × wall height × usable length
Crowned volume = tapered base volume + (top width × crown height ÷ 2 × usable length)
Drive-over crest width = floor width − 2 × side slope × peak height
Daily silage DM = animals × direct DM/head, or animals × direct as-fed/head × DM fraction
Advanced daily silage DM = animals × body weight × DMI fraction × silage share of ration DM
Daily silage DM removed = consumed DM ÷ (1 − feeding-loss fraction)
Packed DM required = daily removed DM × feeding days ÷ (1 − storage-loss fraction)
As-fed silage required = packed DM ÷ silage-DM fraction
Storage volume = as-fed mass ÷ packed as-fed density
Capacity ranges vary only density while holding the measured geometry constant. The calculator assumes a consistent cross-section and density within each matching unit. Excluded ramp length is subtracted before volume is calculated.
Dimensions-First Capacity Example
A tapered corn-silage bunker with a 30 ft floor width, 36 ft top width, 12 ft wall height and 150 ft usable length has 59,400 ft³ of volume. At an entered density of 45 lb as-fed/ft³ and 35% DM, it holds about 1,336.5 U.S. tons as-fed and 467.8 tons DM.
After 15% storage shrink and 8% feeding loss, about 365.8 tons DM remain usable. If daily removal is 12,000 lb as-fed, the inventory lasts about 174 days and the active face advances about 8.1 in/day. Density-range results show why measured cores improve confidence.
Herd-Sizing Example: 200 Animals and Two Bunkers
With 200 animals averaging 1,400 lb, total DMI at 3% of body weight, and this silage providing 50% of ration DM, daily silage consumption is 4,200 lb DM. At 8% feeding loss, 15% storage loss and 35% silage DM, about 2,801 U.S. tons as-fed must be placed for 365 feeding days.
At 45 lb as-fed/ft³ and a 12 in/day target face advance, required volume is about 124,500 ft³ and active face area is about 341 ft². With 12 ft average packed depth, average internal width is about 28.4 ft. Two sequential bunkers would each need about 182.5 ft of usable length.
Calculation Basis and References
- University of Wisconsin Extension: Bunker Silo Sizing documentation — herd silage DM need, separate storage and feeding losses, packed density, moisture, face removal, storage period and practical bunker dimensions.
- University of Wisconsin Extension: Feedout losses from forage storage systems — as-fed density and face-removal guidance, including a 12 in/day design value.
- University of Wisconsin Extension: Packing bunker and pile silos to minimize porosity — minimum density discussion and the relationship between density, dry matter, porosity and loss.
- University of Wisconsin Extension: Drive-over silage pile construction — crop dry-matter ranges, pile geometry, packing layers and storage-loss context.
- Utah State University Extension: Chopping and Storing Quality Corn Silage — the advisory 800 lb packing-weight-per-ton/hour rule.
- University of Minnesota Extension: Hay and silage storage losses — covered bunker moisture and dry-matter loss ranges.
- Penn State Extension: Horizontal silo safety — bunker, pile and feedout hazards and safe-management considerations.
Crop presets synthesize these planning ranges; they are not crop tests. References support the method and advisory thresholds, but do not certify any ration, structure, equipment plan or silage face.
Silage Bunker Calculator FAQs
How many tons does my silage bunker hold?
Multiply the measured internal volume by packed as-fed density, then divide pounds by 2,000 for U.S. tons. Because density varies, this calculator also shows low and high estimates from the selected forage range.
Should I use as-fed density or dry-matter density?
Use packed as-fed density when estimating wet tons from volume. Dry-matter density equals as-fed density multiplied by the forage dry-matter fraction and is useful for comparing packing quality across moisture levels.
Is moisture percentage the same as dry-matter percentage?
No. Dry matter percentage equals 100 minus moisture percentage. A forage at 65 percent moisture is 35 percent dry matter.
How do I measure tapered walls or a crown?
Measure floor and top internal widths separately. For a crown, also measure the wall-level packed depth and the additional average crown height. Exclude ramps, end wedges, and visibly spoiled material from usable dimensions.
How should shrink be included?
Capacity is the packed inventory. Usable inventory is packed dry matter after storage shrink and feeding loss are applied sequentially, not added.
Can this calculator estimate an irregular drive-over pile?
It can approximate a consistent drive-over cross-section from floor width, peak height, side slope, and usable length. Survey irregular piles in multiple slices and total the slice volumes for a better inventory estimate.
Can this calculator approve bunker walls, pile slopes, or packing safety?
No. It is an inventory and planning estimator, not a structural design, ration approval, or safety certification. Obtain site-specific agricultural engineering and safety advice.